FS#36 - Player signals

Implement a system where players can send arbitrary signals to each other(arbitrary, from the point of view of the network protocol). This would be usedto send trade and docking requests, and to replace the silly 'hail' commandwith a 'request for private chat'.

In the future this could also be used for other signals, like players declaringtheir hostile intent.

It might also be possible to use the signals mechanismto exchange targetting info.